Method of bleaching pulp



Patented Feb. 14, 1939 UNITED STATES PATENT OFFICE 2,147,618 METHOD or BLEACHING PULP No Drawing. Application April 1, 1936, Serial No. 72,059. Renewed June 15, 1938 7 Claims.

My present invention relates to improvements in methods of bleaching pulp. More particularly, it relates to the bleaching of so called chemical pulps, which are obtained from wood or other raw cellulosic material by cooking the raw comminuted material with various chemical agents, the more important of such methods being termed the soda, sulphite, sulphate and monosulphite. Pulp so obtained is commonly bleached by the action of chlorine in any one of several forms. As is well known, too drastic action of the bleach on the pulp results in a loss of strength and a deterioration of the fiber. Kraft pulp in particular, which is produced by light or moderate sulphate cock, is of darker color than sulphite or soda pull for example, and is especially difilcult to bleach.

I have now discovered that unexpectedly favorable results are to be had by employing, subsequent to a preliminary chlorine treatment, a treating step in which the pulp in suspension or admixture with water is treated with air or gas containing free oxygen, the pH of the pulp during such treatment being from 8 to 10 or greater. The air or oxygen may either be blown into the pulp suspension, or the water in which the pulp is suspended may have had previously dissolved in it the requisite quantity of oxygen. In carry ing out the invention, the preliminary chlorine treatment may be applied until the pulp no longer absorbs chlorine readily, indicating a satisfactory chlorine treatment, even though the pulp remains yellow in color. The pulp is washed and then made alkaline; i. e., of a pH from 8 to 10 with caustic soda or lime, or with sodium carbonate solution, and then blown with air. The soluble caustic alkalis such as caustic soda or caustic potash produce the best results. The eflect of the oxygen of the air apparently is to oxidize the yellow coloring materials (which are thought to be of the nature of a phlobo-tannin) in the presence of alkali, to form soluble oxidation products which are removed by washing. The improved method will be best understood by reference to the following illustrative example:

Five tons of kraft pulp in aqueous suspension are treated with chlorine in accordance with known practice until a small amount of free chlorine remains present in the pulp suspension after a period of several minutes, indicating a substantially complete chlorination of the pulp. The amount of chlorine employed will vary between 400 and 1200 pounds or more depending on the method by which the kraft pulp is cooked and the wood from which the pulp is made. In

lieu of gaseous chlorine, other chlorination agents may be used such as hypochlorite salts, bleaching powder, etc, provided that the pH of the pulp suspension during the chlorinating treatment is maintained under 6.5 which indicates that the pulp is acid. The chlorinated pulp is preferably washed at this point and the pH value of the stock is then brought to a value greater than 8.0 and preferably as high as 9.0 to 10.0 by the addition of caustic soda solution, 10 whereupon air, oxygen or a gas rich in oxygen is passed through the pulp for a period of from 1 to 4 hours or until no further removal of coloring material is had. The pulp is again washed and is subjected to a bleaching treatment in known manner with a bleaching agent, preferably a hypochlorite salt. This bleaching treatment may preferably be carried out under alkaline conditions; e. g., at a pH greater than 8,

during which air or oxygen may be bubbled through the suspension of pulp in water. After washing the pulp, the bleaching operations may ordinarily be considered to be completed.

However, in cases where pulps are bleached with difficulty to a suflicient degree of whiteness, a second treatment of the pulp may be made with free chlorine under conditions in which the pH of the pulp suspension is less than 6.5; after which the pulp may be washed and rendered alkaline or this washing may be omitted. The treatment with air or oxygen can again be applied to the pulp after it has been adjusted to a pH greater than 8.0 by the addition of caustic soda solution, as in the first treatment by air. After washing the pulp, the bleaching can be completed by means of ahypochlorite in the customary manner or as before at a pH of 8.0, with or without the addition of air to the pulp at this point.

In bleaching other chemical pulps the proce- 40 dure is essentially the same, i. e. a bleach by an agent containing chlorine, or by means of free chlorine itself, washing and an air treatment under alkaline conditions. Any temperature may be employed between 0 C. and 100 C. preference being had for temperatures around 0., at which the reaction ismore rapid than at atmospheric temperatures.

The effect of the air in no way damages the pulp. In one run in which the air bleach was 50 employed a final color was obtained of 261 (Hess- Ives). The same pulp without the air treatment had a color of 252 Hess-Ives), whereas paper made with the same stock with and without the air bleach had substantially the same strengthcharacteristics. On the other hand, ii it had been attempted to raise the color to a value of 261 (Hess-Ives) by the use oi ,chlorine or hypochlorite, the pulp would have been worthless for paper making purposes. 7,

In carrying out my invention it will be understood various changes may be made from those described in detail above, as will be obvious to those skilled in the art. while my invention is especially applicable to chemical pulps it may also be used with advantage in the bleaching oi ground wood when the latter is bleached with chlorine.

I am aware that it has been proposed to blow air through pulp following the chlorination step. In such cases, however, the pulp has been either acid or neutral and consequently the beneficial effect oi my invention was not had.

I claim:

1. The method oi bleaching chemical pulp which consists in subjecting the pulp to the action of chlorine at a pH not substantially in excess of 6, washing the pulp, rendering it alkaline in the range of pH=not less than 8 with a nonoxidizing alkali oi the alkali metal group, then, while the pulp is in suspension'in the dilute alkali, treating it with a gas containing iree oxygen.

2. The method of bleaching chemical pulp which consists in subjecting the pulp to the action of chlorine at a pH not substantially in excess oi 6, washing the pulp, rendering it alkaline in the range of pH=not less than 8 with a non-oxidizing alkali of the alkali metal group, then, while the pulp is in suspension in the dilute alkali, treating it with a gas containing iree oxygen, washing and again bleaching by the use oi a hypochlorite.

3. The method oi bleaching chemical pulp which consists in subjecting the pulp to the action of chlorine at a pH not substantially in excess of 6, washing the pulp, rendering it alkaline in the range oi pH=8 to pH=l0 with an alkali oi the alkali metal group, then, while the pulp is in suspension in the dilute alkali, treating it with a gas containing iree oxygen.

' 4. The method oi bleaching chemical pulp which consists in subjecting the pulp to the action oi chlorine at a pH not substantially in excess of 6, washing the pulp, rendering it alkaline in the range of pH=not less than 8 with a non-oxidizing alkali oi the alkali metal group, then, while the pulp is in suspension in the dilute alkali, treating it with a gas containing iree oxygen, washing and again bleaching by the use oi a hypochlorite under alkaline conditions and simultaneously treating the pulp with a gas containing iree oxygen.

5. The method oi'bleaching unbleached chemical pulp, which consists in subjecting the pulp to the action oi chlorine at a pH not substantially in excess of 6, washing the pulp, rendering it alkaline in the range oi pH=not less than 8 with a non-oxidizing alkali, then while the pulp is in suspension in the dilute alkali, treating it with a gas containing iree oxygen.

6. The method oi bleaching unbleached chemical pulp, which comprises subjecting the pulp to the action of chlorine at a pH not substantially in excess of 6, rendering the pulp alkaline in the range of pH=not less than 8, and then, while the pulp is in the alkaline suspension, treating it with a gas containing iree oxygen.

7. The method of bleaching unbleached chemical pulp, which comprises subjecting the pulp to the action of chlorine at a pH not substantially in excess of 6, rendering the pulp alkaline in the range of pH=not less than 8, and then, while the pulp is in the alkaline suspension, treating it with a gas containing iree oxygen, and bleaching the pulp so treated with alkaline hypochlorlte solution.

FRANCIS G. RAWLING. 

